1. Field of the Invention
The present invention relates to a variable linearity coil which is used in a monitor of a personal computer, or the like.
2. Description of the Related Art
As is well known, the linearity coil is interposed for connection to a horizontal deflection circuit of a monitor of a television receiver set, personal computer, or the like for the purpose of correcting distortions on a monitor screen. In the television receiver set, or the like, the frequency of the electric current to flow or to be charged in the linearity coil is constant at 15.75 kHz or 33.75 kHz. Therefore, the correction of the distortions was possible with a linearity coil of a constant or predetermined DC (direct current) magnetic field. In the monitor of the personal computer, on the other hand, the range of the frequency of the electric current to be charged is as wide as 15 kHZ to 120 kHz. Therefore, there occurs a difference in the amount of correction between the time when the frequency is low and the time when the frequency is high. It follows that an appropriate correction of distortions cannot be made in a predetermined DC magnetic field.
As a solution, there has conventionally been proposed the following variable linearity coil as shown in FIG. 6. Namely, a magnetic core b which is made by winding therearound a coil "a" and a magnetic core d which is made by winding therearound a bias magnetic field adjusting coil c are placed one on top of the other in a vertical posture (i.e., with an axis of winding the coil extending in the up and down direction) together with a permanent magnet e. The sub-assembly thus obtained is placed on an electrically insulating base g which is made, for example, of a resin and which has embedded therein lead terminals f. The lead terminals f are connected to terminal ends of respective winding coils of the coil "a" and the bias magnetic field adjusting coil c.
In the above-described conventional variable linearity coil, when a sawtooth wave electric current is charged to the coil "a" which is interposed for connection to a horizontal deflection circuit of a cathode ray tube of a television set or the like, the magnetic core b will give rise to magnetostriction vibrations. The vibrations are then transmitted to a printed-circuit board via the base g, resulting in a resonance of the printed-circuit board. Consequently, beat notes are sometimes generated or, even if beat notes are not generated, the vibrations are transmitted to other component parts, resulting in a loss in their reliability. Further, since the magnetic coupling between the coil "a" and the bias magnetic field adjusting coil c is high, the sawtooth electric current in the coil "a" is induced to the bias magnetic field adjusting coil c. The sawtooth wave electric current thus flows in a manner overlapped with the DC bias control current of the bias magnetic field adjusting coil c. Therefore, a predetermined magnetic bias cannot be given to the core b. As a consequence, the original (or inherent) characteristics of the linearity coil cannot be obtained and the image on the cathode ray tube is thus disturbed.
In view of the above-described problems with the conventional variable linearity coil, the present invention has an object of providing a variable linearity coil: in which, when surface-mounted on a printed-circuit board, the occurrence of magnetostriction vibrations can be prevented; in which the reliability of other parts which are surface-mounted on the printed-circuit board is not impaired; and in which inherent characteristics of the linearity coil can be obtained to thereby cause no disturbances in the image on the screen of the cathode ray tube, or the like.